1. Field of the Invention
The present invention relates to a variable displacement pump useful as a pressure supply source of a pressure fluid utilization apparatus such as a power steering gear for the vehicle.
2. Description of the Related Art
A variable displacement pump of vane type typically includes a cam ring having a cam face on the inner circumference thereof, a rotor rotating within the cam ring, a plurality of vanes inserted retractably into slits formed at regular intervals circumferentially on the outer circumference of the rotor, and two plates (or plate-like pump bodies) carrying the cam ring and the rotor from both sides. Each vane slides with the cam ring along with the rotation of the rotor to increase or decrease the volume of a pump chamber formed between two adjacent vanes, so that oil is sucked or discharged.
In this vane pump, a back pressure inlet bore with an inner circumferential end portion of each slit expanded is provided so that each vane is pushed out of the slit of the rotor and surely contacted with an inner circumferential cam face of the cam ring, and a circular groove opposed to the back pressure inlet bore is formed on a face of the plate contact with the rotor. An oil discharged from the pump is introduced via this groove into the back pressure inlet bore.
The vane pump according to the related art applies a pump discharge pressure to a base end portion (an end portion on the inner side) of the vane so that the vane is pushed out and surely contacted with the cam. Therefore, oil must be discharged excessively by the amount needed to push out the vane, and if the discharge pressure is increased, a vane located in a suction area at low pressure is always pressed strongly against the cam, increasing a friction loss, so that the drive power of the pump is increased due to the increased discharge amount and friction loss, leading to a problem of worse fuel consumption. Also, the cam contact portion is worn due to friction, leading to a problem of shorter life.
In the constitution according to the related art, a discharge pressure is introduced into the bottom portion of slit so that the vane is pushed out and pressed against the cam, whereby the discharge amount is increased, and the suction area vane is pressed more strongly than needed, resulting in the above problems. Thus, a variable displacement pump has been proposed in which the circular grooves are formed in the discharge area and the suction area, respectively, to introduce a pump discharge pressure into the discharge area groove, and introduce a pump suction pressure into the suction area groove (JP-A-6-200883 and JP-A-6-241176).
In the constitution of the variable displacement pump as disclosed in the above publications, a pressure almost equal to the pressure within the pump chamber facing a top end portion of the vane is introduced into the base end portion of vane, resulting in a problem that a pressing force of the vane against the cam ring is insufficient.